PURPOSE: To prevent water leakage by supporting a wall panel and a window glass panel to wall panel supporting mullions arranged side by side at predetermined intervals and by providing a water drain mechanism in an equally pressurized cavity in wall.

CONSTITUTION: Each wall panel 11 is equipped with a front metal finish plate 19, an upper outer peripheral member 20 consisting of an aluminum member, and a lower outer peripheral member 21, and the upper outer peripheral member 20 is fitted to the surface of the mullion 14 with aluminum clips. And a split muff coupling spline member 24 in horizontal direction is fixed by snapping to the upper outer peripheral member 20, and the front plate 19 is sealed by caulking 31, thereby using it as a rain screen in a horizontal direction. Next, water running on the surface of this rain screen in horizontal direction is guided to an external gutter 26, water flows from the drain hole 38 to a drain tunnel 27 and is discharged to a downspout groove. Thus, water from the outside does not invade a seal type wall joints thereby preventing water leakage.